linux-kernel - Re: [PATCH v4 3/4] cachestat: implement cachestat syscall

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Date:   Fri, 16 Dec 2022 13:48:14 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Nhat Pham <nphamcs@...il.com>
Cc:     hannes@...xchg.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, bfoster@...hat.com,
        willy@...radead.org, kernel-team@...a.com
Subject: Re: [PATCH v4 3/4] cachestat: implement cachestat syscall

On Fri, 16 Dec 2022 11:21:48 -0800 Nhat Pham <nphamcs@...il.com> wrote:

> Implement a new syscall that queries cache state of a file and
> summarizes the number of cached pages, number of dirty pages, number of
> pages marked for writeback, number of (recently) evicted pages, etc. in
> a given range.
> 
> NAME
>     cachestat - query the page cache status of a file.
> 
> SYNOPSIS
>     #include <sys/mman.h>
> 
>     struct cachestat {
>         __u64 nr_cache;
>         __u64 nr_dirty;
>         __u64 nr_writeback;
>         __u64 nr_evicted;
>         __u64 nr_recently_evicted;
>     };
> 
>     int cachestat(unsigned int fd, off_t off, size_t len,
>           size_t cstat_size, struct cachestat *cstat,
>           unsigned int flags);
> 
> DESCRIPTION
>     cachestat() queries the number of cached pages, number of dirty
>     pages, number of pages marked for writeback, number of (recently)
>     evicted pages, in the bytes range given by `off` and `len`.

I suggest this be spelled out better: "number of evicted and number or
recently evicted pages".

I suggest this clearly tell readers what an "evicted" page is - they
aren't kernel programmers!

What is the benefit of the "recently evicted" pages?  "recently" seems
very vague - what use is this to anyone?

>     These values are returned in a cachestat struct, whose address is
>     given by the `cstat` argument.
> 
>     The `off` and `len` arguments must be non-negative integers. If
>     `len` > 0, the queried range is [`off`, `off` + `len`]. If `len` ==
>     0, we will query in the range from `off` to the end of the file.
> 
>     `cstat_size` allows users to obtain partial results. The syscall
>     will copy the first `csstat_size` bytes to the specified userspace
>     memory. `cstat_size` must be a non-negative value that is no larger
>     than the current size of the cachestat struct.
> 
>     The `flags` argument is unused for now, but is included for future
>     extensibility. User should pass 0 (i.e no flag specified).

Why is `flags' here?  We could add an unused flags arg to any syscall,
but we don't.  What's the plan?


Are there security implications?  If I know that some process has a
file open, I can use cachestat() to infer which parts of that file
they're looking at (like mincore(), I guess).  And I can infer which
parts they're writing to, unlike mincore().


I suggest the [patch 1/4] fixup be separated from this series.